This invention relates to the injection of gaseous fuel into the combustion chamber of a gas engine. The injection of gas into a combustion chamber of current designs of gas engines occurs through the different types of orifices under pressures (Pinj) that range xcx9c40-60 Psi. The pressure in cylinder (Pcyl) during the injection process is changing from xcx9c20 to 60 Psi. The differential pressure dPPinlxe2x88x92Pcyl, is small and consequently gives a small velocity of fuel jet and poor mixing. Some existing designs use high pressure, but still use orifices or pipe (constant area duct) for the nozzle. The flow in these orifices is choked flow, and as such, the velocity of fuel jet never exceeds the speed of sound. This speed of sound is calculated based on the local parameters in the outlets. So, there is a limitation of the velocity, and mixing, in existing designs. Thus, mixing is poor.
The present invention contemplates the use of an injection pressure that is always higher than maximum pressure in the cylinder at the end of the injection process. This pressure should be at least Pinj=85 Psi, for Pcyl=53 Psi. By using this level of injection pressure, it is possible to realize supersonic flow. The design of this invention is a profiled nozzle, which accelerates the flow to above Mach 1. As long as the pressure ratio satisfies Pinj/Pcyl greater than 1.592 (for the Natural gas, Cp/Cv=1.31), supersonic flow can be achieved. To create such flow, a special profiling of the nozzle area is required. Gas dynamics theory described, for example, by Philip Thompson (Philip Thompson. Compressible-Fluid Dynamics, Department of Mechanical Engineering, Rensselaer Polytechnic Institute (1988), pp. 665) is used to profile the nozzle area. Generally, the nozzle will have a converging-diverging shape. For given pressure on the inlet there is acceleration of the flow in the converging part of the nozzle. At the minimum area of the nozzle, the critical area, the flow will reach maximum velocity which is still not more than sonic velocity. To accelerate the flow further, the nozzle must then begin to diverge. Applying the gas dynamic theory, the throat area can be calculated that is required to provide the required amount of fuel. For this the parameters on the nozzle inlet (pressure Pinl, temperature Tinl, velocity Vinl) are used. Then, for known parameters of throat and designed outlet pressure Pout, the outlet area is defined. The ratio of local gas flow speed to speed of sound (Mach number, M=V/C) in outlet will depend on the throat to the outlet area ratio (AR=A*/Aout). It is recommended Mxcx9c2.5-3. In this case, the flow on the outlet will be supersonic. Depending on the ambient pressure at the outlet, the flow could be over-expanded. If it is over-expanded, there will be shock waves, after which the pressure will take the value of ambient pressure.
The first advantage is with a supersonic injector the jet flow velocity on outlet of the nozzle is the maximum for a given pressure on the inlet. It could be higher than the speed of sound, and much higher than for conventional injection devices.
Second, the high velocity of the jet creates highly turbulent flow and better entrapment of the cylinder air into the jet of gaseous fuel. This is due to interface instability of the jet in ambient air.
Third, this supersonic injector could be used in gas turbine engine which use the gaseous fuel.
Fourth, in the case of over-expanded flow, the existence and special configuration of shock waves will improve the mixing through increasing entropy and micro-scale vortex formation in the shock waves. The vortexes improve the mixing in a larger area and consequently make the combustion mixture more uniform. This mechanism is an additional to the interface instability.
Fifth, multiple supersonic nozzles for one fuel valve are used. The interaction of multiple jets radically increase the mixing of fuel and air in cylinder.
Sixth, the combined advantages allow preparing more uniform mixture for combustion, reducing the temperature and improving the combustion. As a result, for the lower injection pressure (100-150 Psi) it makes it possible to reduce Nox and increase the engine efficiency.
The subject of this invention is the design and use of a supersonic injector for a gas engine.